The present invention relates to a method for cleaning unbrazable metal parts sufficiently to render them brazable and more particularly, to a method for the low temperature cleaning of such parts with an atmosphere having the elements H--O--C--F so that the parts can be, thereafter, brazed or otherwise bonded.
Late model gas turbine engines for example, those of the Boeing 747, the DC--10, and the Lockheed 1011) employed in their turbine sections nickel based alloys that are gamma prime hardened. Many other engines contain such materials, and the use of gamma prime hardened alloys will no doubt increase in the future due to the desirable properties of these superalloys.
The problem is that engine parts made of such alloys are very expensive and, at present, are not repairable when crack damaged due to metal fatigue. Attempts to weld repair such components results in post-weld cracking. Likewise, conventional brazing cannot be affected because nickel-base braze alloys will not run on the gamma prime hardened surfaces. Thus, although a molten brazing alloy under high vacuum might stick new gamma prime hardened parts together, it has not previously been possible to place brazing alloy inside cracks in damaged gamma prime hardened alloy parts.
A successful braze is manifest when braze material is placed at the source of a crack (say 0.001 inch wide and one-half inch long) and, at brazing temperature, not only melts and sticks to the parent material, but also runs into and fills the length of the crack. Apparently, in use a gamma prime hardened alloy becomes oxidized (and/or sulfuridized) to the extent that the aluminum, titanium and chromium oxides (or sulfides) which coat the surface of the part, including the surfaces of the crack, prevent successful repair by brazing.
Accordingly, it has been recognized that such parts must be cleaned if they are to be brazed. One suggestion is to use chromium fluoride (CrF.sub.3) and hydrogen (H.sub.2) to clean damaged parts of gamma prime hardened alloys prior to a braze repair. It is speculated that the following reaction mechanism takes place: ##STR1## (2) MO.sub.x +HF.fwdarw.MF.sub.x +H.sub.2 O+M If, then, MF.sub.x is volatile at the reaction temperature, the oxide is effectively reduced and the base metal (M) should be brazable. However, uniform reproducibility of results is for some reason lacking and many parts cleaned by this process are still not brazable.
A much more effective process is that disclosed and claimed in parent application Serial No. 874,915. However, that application is for the most part directed to cleaning crack-damaged gamma prime hardened alloys. It has now been established that a similar process is also effective in cleaning other metals, especially stainless steels, superalloys, and solid solution superalloys, as well as the gamma prime hardened nickel alloys, see companion application Ser. No. 119,060, filed on Feb. 8, 1980 an even date herewith. However, the temperature required by all prior cleaning processes is relatively high.
Fabrication of stainless steel composites by nickel brazing has long been of commercial interest. Before such devices can be fabricated by brazing, however, it is necessary to clean the faying surfaces of all metal oxides (or other compounds). The problems arises because the surfaces of such alloys are covered with a passive film which will not be wetted by a brazing alloy. The most stable oxide in such a film is that of chromium, and any pre-braze cleaning technique necessarily centers on this compound. It is necessary to reduce the chromium oxide (and all other oxides) to its metallic element before brazing can be accomplished.
A technique that is commonly employed to prepare such alloys for brazing is that of exposing them to a dry hydrogen atmosphere at high temperatures (&gt;1000.degree. C.). Hydrogen cleaning is highly functional but has the disadvantage that the cleaning only takes place at temperatures that are near or higher than the brazing temperature. Therefore, in situ cleaning and brazing of stainless steel and superalloy assemblies is often not practicable.
It is known that stainless steel can be brazed in a stable reducing atmosphere of fluoride. In a paper presented by the Toulouse, France, Microturbo Company representatives at the American Welding Society (AWS) meeting in Philadephia in April 1977, entitled "Brazing Stainless Steel in a Stable Reducing Atmosphere of Fluoride," there is described a brazing process carried out in a halogen atmosphere obtained by the decomposition of fluorine salts such as ammonium bifluoride acid and chromium fluoride. The proposed reactions are: ##STR2## (2) NH.sub.3 .fwdarw.1/2N.sub.2 +3/2 H.sub.2 (on contact with metal) (3) CrF.sub.3 +H.sub.2 .uparw..fwdarw.2HF+.uparw.Cr
(4) 6HF+Cr.sub.2 O.sub.3 .fwdarw.2CrF.sub.3 +H.sub.2 .uparw.+F.sub.2 .uparw.
There are two pertinent observations regarding these reactions: (a) the object would appear to be the production of HF gas which, in turn, does the cleaning, and (b) any elemental fluorine that forms is produced downstream of the work piece (see reaction 4). It is noted that in the presented paper there is an indication that "the technique cannot be used on assemblies of materials having a high level of electroposivity, such as titanium and zirconium," and "it is essential to avoid the introduction of carbon into the furnace during brazing."
Similarly, Moore in U.S. Pat. No. 3,585,819, discloses a process of fluxing metal parts with a stable, non-oxidizing atmosphere containing HF gas. The metal parts are ones such as steels which are to be brazed or soldered.
Finally, reference is made to Low U.S. Pat. No. 2,851,387. Low relates to a process for nitriding high chromium stainless steels. In Low's discussion of the prior art he notes that all prior processes of depassifying such steels require immediate nitriding or the internal affects of the depassifying were lost. A specific purpose of his invention is a combined activating and nitriding operation which avoids any problems of interruption in the sequence steps. The combined operations are provided by a mixture of decomposed fluorocarbon resin gases and ammonia gas. The result is a nitrided product, but it is believed that a cleaned, brazable product would not be produced.
Accordingly, the need steel exists for relatively low temperature method for cleaning metal parts to render them brazable or otherwise bondable.